lnrouter.get_distances: fix exception due to rare race
lnrouter.get_distances: fix exception due to rare race

if the graph is being updated while the pathfinding is running,
channel_info might be None here

spesmilo/electrum,wakiyamap/electrum-mona,spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,wakiyamap/electrum-mona,pooler/electrum-ltc,pooler/electrum-ltc,wakiyamap/electrum-mona,spesmilo/electrum,wakiyamap/electrum-mona,pooler/electrum-ltc